SPEAKER OHMS/WATTAGE

Instead of using all the 5 speakers that come with my Sony DVD/VCR RECEIVER HOME THEATRE SYSTEM, I would like to substitute the 2 rear speakers with a SANSUI SP1200SPEAKER. THE TWO REAR SPEAKERS OF THE SONY IS; SONY SPEAKER MSP76, RATED AT 6 OHMS, 70 WATTS

I LIKE TO CHANGE IT WITH 2 SANSUI SP1200 SPEAKERSWHICH IS A 3 WAY, 5 SPEAKER RATED AT 8 OHMS, 60 WATTS.

Before I do the change, can anybody please tell me what to expect? 1. Will it significantly change the overall sound quality? 2. will i damge the system because of the change in watts and ohms in the speakers?